1. Field of the Invention
This invention relates to search, extrication and retrieval of foreign objects from a heat exchanger and more particularly to positioning search, extrication and retrieval devices for such objects within a nuclear steam generator. The invention also relates to positioning such devices within a nuclear steam generator using a sludge lancing system.
2. Background of Information
In a pressurized water nuclear powered electric generating system, the heat generated by a nuclear reactor is absorbed by a primary coolant that circulates through the reactor core and is utilized to generate steam in a steam generator. The steam generator typically is an upright cylindrical pressure vessel with hemispherical end sections. Such a generator typically comprises an outer vertically oriented shell, a horizontal plate called a tube sheet adjacent to the lower end of the shell, a bundle of vertical U-shaped tubes supported by the tube sheet, and a wrapper barrel inside of the outer shell surrounding the tubes and extending from the upper portion of the shell downwardly to a predetermined point above the tube sheet. The wrapper barrel forms a narrow annulus inside the shell and generally extends down to a point approximately twelve to fourteen inches above the tube sheet. The outer cylindrical shell is provided with one or more openings of limited size called handholes which are typically located about five to twenty-one inches above the tube sheet. These handhole openings are covered during operation of the steam generator but may be opened when the generator is shut down to permit access to the area inside for maintenance purposes.
Occasionally, during maintenance inside the steam generator, objects such as bolts, wires or other foreign objects are inadvertently introduced into the system and have to be removed. Due to the limited space within the generator, the annulus between the wrapper barrel and the shell generally is only about 1.5 to 2.75 inches wide, the space between the shell and the tubes is typically only about 4.5 inches wide, and the space between the bottom of the wrapper barrel and the tube sheet is usually only about twelve to fourteen inches high. The tubes in the tube bundle are typically spaced about 0.292 to 0.406 inches apart. Various objects may easily become entrapper between these closely spaced tubes. Therefore, it is difficult to locate, dislodge or remove such objects between the tubes on the tube sheet.
U.S. Pat. No. 5,065,703 discloses a flexible lance for use in a steam generator tube bundle. The flexible lance utilizes high-pressure water jets, a video camera and a sludge sample retrieval mechanism. The flexible lance includes a plastic extrusion having a plurality of conduits extending lengthwise along such plastic extrusion. The conduits have a helically wound cartridge brass core covered with a braided brass sheath. A flexible cable is movable within the plastic extrusion to operate a sludge sampler. A fiber optic cable is connected to inspecting optics in order to view the interior of the steam generator. The fiber optic cable, the plural conduits, and the flexible cable are routed through the longitudinal axis of the plastic extrusion.
U.S. Pat. No. 4,638,667 discloses a probe positioning apparatus which includes an elongated extensible boom and an elongated flexible probe carrier tape. The boom has an end which is adapted for extension into and retraction from the tube lane of a steam generator. The carrier tape has sprocket holes for transporting the probe. The end of the boom, which is located within the steam generator, has a tractor feed unit and two sprocket belts for feeding the carrier tape. A drive motor rotates the tractor feed unit 180.degree. in order to investigate tube rows on both sides of the tube lane. Located outside of the steam generator is a tape withdrawing assembly which includes a drive motor and a sprocket for retrieving the carrier tape.
U.S. Pat. No. 4,760,876 discloses a transport system for an inspection system having an end effector which carries inspection and gripping devices, a hollow flexible conduit which passes control cables for the end effector therethrough, and a drive assembly which moves the flexible conduit in axial and rotational directions.
U.S. Pat. No. 4,702,878 discloses a device for searching and retrieving foreign objects on the tube sheet of a steam generator. The device includes a probe for searching for foreign objects on the surface of the tube sheet and a gripper for grasping one of the foreign objects. The device is inserted into the annulus using a guide tube having a straight main segment, an L-shaped upper segment which is bent at a right angle with respect to the main segment, and an L-shaped lower segment. The lower segment, which rests on the surface of the tube sheet, is bent at a right angle with respect to the main segment and extends in a direction rotated 90.degree. from the direction of the upper segment. The upper segment is secured to the handhole by a guide plate. To facilitate insertion of the guide tube into the steam generator, the upper, main and lower segments are constructed from separate parts.
U.S. Statutory Invention Registration No. H1115 discloses a robot arm apparatus having two or more cascaded conduit elements and a flexible movable conduit. The flexible conduit passes through the cascaded conduit elements and conveys inspection and/or maintenance apparatus to the interior of a steam generator. The flexible conduit has a terminal working end which is translated into and around the interior of the steam generator. A first reversible motor translates the cascaded conduit elements in a first axis within the tube lane. A second reversible motor translates the flexible conduit in a direction perpendicular to the first axis. A third reversible motor rotates a carnage which carries the cascaded conduit elements and, thus, moves the terminal working end in a third direction of travel, which is a curved path.
It is known to manually utilize a pair of parallel guide tubes between one handhole and the tube lane in order to manually and independently position, through each of the parallel guide tubes, a search probe and a retrieval device. Such parallel guide tubes have an L-bend in order to manually and independently position, through each of the parallel guide tubes, a search probe and a retrieval device in an axis perpendicular to the tube lane. It is also known to manually utilize two guide tubes between one handhole and the annulus. These two guide tubes are used to manually and independently position a search probe and a retrieval device on the tube sheet within the annulus.
The handholes also provide access to the tube sheet for removal of sludge deposits on the tube sheet. An apparatus and method for removing such sludge from a steam generator are described in U.S. Pat. No. 4,079,701 entitled "Steam Generator Sludge Removal System," issued Mar. 21, 1978, and in U.S. Pat. No. 4,276,856 entitled "Steam Generator Sludge Lancing Method," issued Jul. 7, 1981, which are both incorporated herein by reference.
During a sludge lancing operation, a fluid injection header and a fluid suction header are placed at opposite handholes near the elevation of the cylindrical tube sheet. These cause a circumferential fluid stream to be established from the injection header around the tube bundle to the suction header. A tubular member having a rack is moved by a gear along the tube lane between the injection header and the suction header. The tubular member includes a head having two or more nozzles which emit pulsating fluid jets substantially perpendicular to the line of travel along the tube lane. In this manner, the pulsating fluid jet forces sludge to the periphery of the cylindrical tube sheet where the sludge is entrained in and carried away by the circumferential fluid stream.
After a sludge lancing operation, a cleanliness inspection must be performed. Subsequently, the conventional sludge lancing equipment is removed and the conventional inspection and/or retrieval equipment is installed. In the event that the steam generator is not cleaned satisfactorily, the inspection and/or retrieval equipment is removed and the sludge lancing equipment is reinstalled and the process is repeated. However, during such removals and installations, sludge lancing and inspection personnel for nuclear steam generators are exposed to increased levels of radiation at the handhole.
There is a need, therefore, for an apparatus which permits a cleanliness inspection to be performed without unnecessarily exposing inspection personnel to radiation.
There is a more particular need for such an apparatus which permits foreign objects discovered by the cleanliness inspection to be retrieved in conjunction with the inspection.
There is also a need for an improved sludge lancing apparatus which permits a cleanliness inspection to be performed without unnecessarily exposing inspection personnel to radiation.